Dystroglycan is a cell adhesion molecule that plays important roles in neuromuscular and cortical development and in muscular dystrophy. The glycosylation of a dystroglycan (aDG) is essential for its function; Six genes have been identified that, when mutated, cause the underglycosylation of aDG leading to diminished or absent binding of extracellular matrix proteins, including laminin. Defects in these genes also cause forms of congenital muscular dystrophy (CMD) where underglycosylation of aDG is causative molecular defect. Several of these genes (POMT1, POMT2, POMGnT1) are known to be glycosyltransferases involved in the synthesis of O-linked mannose structures on the aDG protein. In mammals, O-linked mannose has only been shown to exist on aDG. Thus, these carbohydrate structures are highly specific to this one protein. The functions of the three other known CMD genes that alter aDG glycosylation are unknown, despite intense efforts by numerous investigators. One of these is LARGE, the gene deleted in the myodystrophy (LARGEmyd) mouse. LARGE is important not only because defects in this gene cause human disease, but also because its overexpression has been shown to stimulate glycosylation of aDG and rescue laminin binding in cells from CMD patients. Thus, understanding LARGE function may be a key to the development of therapeutics for multiple forms of CMD as well as to understanding the role of dystroglycan in neuromuscular development. Here we provide evidence that suggests that LARGE synthesizes a novel carbohydrate structure. This structure, like O-mannose, appears to be extremely rare in mammals but common in lower organisms. This proposal will conclusively identify the presence of this carbohydrate structure on dystroglycan, define its role in ligand binding, and define the function of LARGE as the enzyme that creates it. This proposal, therefore, will not only define a novel function for a therapeutically important enzyme, but likely identify a glycan structure not yet described in mammals. [unreadable] [unreadable]